You can activate Liliana’s first ability even if some or all players will be unable to discard a card.

12/7/2018

When Liliana’s first ability resolves, first the player whose turn it is chooses a card in hand without revealing it, then each other player in turn order does the same. Then all the chosen cards are discarded at the same time.

12/7/2018

When Liliana’s third ability resolves, you put each permanent the player controls into one of the two piles. For example, you could put a creature into one pile and an Aura enchanting that creature into the other pile.

12/7/2018

A pile can be empty. If the player chooses an empty pile, no permanents will be sacrificed.